This invention relates to optical diffusers which may advantageously be used for front or rear projection screens in general, and in particular to optical diffusers comprising incompatible materials which are mixed in the fluid phase to subsequently form a light diffusive material in sheet form upon conversion to the solid phase.
The underlying mechanism by which projection screens work relies on a material which can scatter the projected image into the viewer's eye to produce what is called a real image. Each dot on the screen acts as a point source of light and all together they produce the images we see. A white screen scatters the light equally in all directions (i.e., Lambertian screen) and thus the image appears equally bright from all vantage points in front of the screen (The extent to which a projection screen can be viewed in the horizontal and vertical planes is generally known or referred to as angle of view (AOV)). A Lambertian screen would thus have an angle of view of 90.degree.. However, the actual brightness (brightness is technically referred to as gain) is very low, since the light is scattered equally in all directions. In typical viewing situations, it is not necessary to project the image to the ceiling nor to the floor since it is unlikely that a viewer will be in those positions. In principle, the brightness of an image on a projection screen from any particular viewpoint can be increased by having the screen direct light preferentially towards that viewpoint rather than scattering light equally in all directions. Therefore, if one could take all the light that would otherwise be wasted in non-viewing regions and re-direct it to the viewing region, i.e., in front of the screen at eye level, this would make the viewing region have much more gain since the extra light is directed there at the expense of the non-viewing region.
Projection screens with such optical properties are referred to as asymmetric screens, i.e., they have an asymmetric "viewing cone", and diffusers with similar characteristics are known as asymmetric diffusers.
For applications such as front and rear television projection systems and brightness enhancing films for computer screens, the importance of obtaining a high gain image over a wide range of horizontal angles but over a narrower vertical angle of view (or, in other words, redirecting the light from the vertical to the horizontal) has been increasingly recognized. In practical terms, this means that the projected image obtained is as bright as possible, regardless of where one is seated in front of the screen. Various solutions to this problem have been presented in the art, such as lenticular lens arrays, embossed screens, etc.
A material available from Rohm and Haas (Philadelphia, Pa.) called PLEX-L.TM., made in accordance with U.S. Pat. Nos. 5,237,004 and 5,307,205, attempts to address some of the performance challenges for projection screens. The product is composed of crosslinked acrylic spherical particles dispersed in an acrylic matrix. The refractive index differential between the spherical particles and the matrix create the light redirecting action via refraction. AOV is thus controlled via the concentration of the spherical particles in the matrix, their .DELTA.n (i.e., differential index of refraction) with respect to the matrix, the particle size and the overall thickness of the film as Matsuzaki et al. teach in U.S. Pat. No. 5,196,960. However, the PLEX-L material has a number of limitations which hinders its wide use in projection screens. As can be seen in FIG. 1, the AOV of PLEX-L varies with the thickness of the screen. A thicker film contains more spherical particles, and hence more scattering centers which lead to a higher angle of view of the resultant screen. However, the increase in screen thickness results in a loss of image definition. Also, it is desirable for a screen to have a high AOV as well as a high gain. These properties, however, are inversely related: high AOV screens tend to be relatively thick and have poor definition and lower gain, whereas low AOV screens tend to be thinner and have better definition and higher gain. It was found that elliptically-shaped particles can be used to create asymmetric optical properties by reducing the vertical AOV while keeping horizontal AOV constant, as described in U.K. Patent No. 540,567 and U.S. Pat. Nos. 4,165,153 and 4,983,016. In U.S. Pat. No. 5,473,454, it was noted that the PLEX-L material can be stretched, causing the spherical particles to attain an elliptical or ovoidal shape and thus preferentially refract the light in the horizontal direction at the expense of the vertical direction. The '954 patent teaches heating a thin sheet of PLEX-L material and stretching it in one direction while the maintaining the dimensions in the transverse direction. The choice of acrylic as the base material, however, is found to limit that stretch to a 2:1 aspect ratio of the spherical particles, which only enhances the gain slightly. Furthermore, the additional and separate stretching step increases the complexity of the manufacturing process, thus limiting the versatility of the end product.